Capacitively measuring sensor and readout circuit

Abstract

A capacitance to be measured is connected together with three further capacitances to form a bridge circuit, in which each two of the capacitors are connected in series, and these two series circuits are connected in parallel to one another, and which replaces a part of the input stage of a Σ-Δ modulator. As inputs of this bridge circuit, terminals are provided at the ends of these series circuits, and as outputs terminals are respectively provided between the capacitors connected one after the other in series. In a sensor realized thereby, two capacitively measuring sensor elements of the same type are used. These capacitors are built into the bridge circuit together with two equally large reference capacitors in such a way that exactly one measurement capacitor and one reference capacitor is La connected is connected to each input and to each output of the bridge circuit.

What is claimed is: 
     
       1. A readout circuit for capacitively measuring sensors, comprising: in an input stage of a Σ-Δ modulator, a bridge circuit having four capacitors;   said bridge circuit having two inputs and two outputs;   in each pair of one of said two inputs and one of said two outputs, respectively one of the four capacitors is connected between the input and the output of said each pair; and   switches and a clock control with which two different potentials are applied in temporally alternating fashion to the two inputs of the bridge circuit.   
     
     
       2. The readout circuit according to claim 1, wherein the four capacitors are formed by two pairs of capacitors with the same capacitance,   wherein capacitors of each of said pairs are connected with different inputs and different outputs of the bridge circuit,   wherein two balancing devices are present that respectively have a number of capacitors and switches, and   wherein the capacitors of said balancing devices are connected in parallel to one another with one of their terminals connected respectively to a common output of the bridge circuit, and are connectable alternatively via the switches to one of the inputs of the bridge circuit with their respective other terminal.   
     
     
       3. The readout circuit according to claim 1, wherein the four capacitors have the same capacitance. 
     
     
       4. The readout circuit according to claim 1, wherein two sensor elements of a common type, each forming a measurement capacitor, form a sensor,   wherein capacitances of said measurement capacitors change in a common direction, given a change in a quantity to be measured,   wherein said measurement capacitors ard connected as two of the four capacitors of the bridge circuit, and   wherein the measurement capacitors are connected with different inputs and different outputs of the bridge circuit.   
     
     
       5. The readout circuit according to claim 4, wherein two reference elements of a common type are present, each of which forms a reference capacitor whose capacitance does not change given a change in a quantity to be measured,   wherein said reference elements are structured similarly to the sensor elements, and are fixed such that capacitances of the reference capacitors are at least approximately equal to capacitances of the measurement capacitors in a predetermined basic state of the sensor elements, and   wherein said reference capacitors are connected as two of the four capacitors of the bridge circuit.   
     
     
       6. The readout circuit according to claim 4, wherein the sensor is a pressure sensor, wherein each sensor element has a membrane that is at least region-by-region electrically conductive, which membrane is deformable for a pressure measurement, and which is accessible with one surface to a medium whose pressure is to be measured, and which forms a part of a measurement capacitor. 
     
     
       7. The readout circuit according to claim 4, wherein the sensor is an acceleration sensor, wherein each sensor element has an at least region-by-region electrically conductive frame part that is movable for an acceleration measurement, and that is maintained in a rest position by a spring-loaded suspension mount in the absence of inertial forces, and that forms a part of a measurement capacitor. 
     
     
       8. The readout circuit according to claim 1, wherein two sensor elements of a common type, each forming a measurement capacitor, form a sensor,   wherein capacitances of said measurement capacitors change in opposite directions, given a change in a quantity to be measured,   wherein said measurement capacitors are connected as two of the four capacitors of the bridge circuit, and   wherein the measurement capacitors are connected to a common output of the bridge circuit.   
     
     
       9. The readout circuit according to claim 8, wherein two reference elements of a common type are present, each of which forms a reference capacitor whose capacitance does not change given a change in a quantity to be measured,   wherein said reference elements are structured similarly to the sensor elements, and are fixed such that capacitances of the reference capacitors are at least approximately equal to capacitances of the measurement capacitors in a predetermined basic state of the sensor elements, and   wherein said reference capacitors are connected as two of the four capacitors of the bridge circuit.   
     
     
       10. The readout circuit according to claim 8, wherein the sensor is a pressure sensor, wherein each sensor element has a membrane that is at least region-by-region electrically conductive, which membrane is deformable for a pressure measurement, and which is accessible with one surface to a medium whose pressure is to be measured, and which forms a part of a measurement capacitor. 
     
     
       11. The readout circuit according to claim 8, wherein the sensor is an acceleration sensor, wherein each sensor element has an at least region-by-region electrically conductive frame part that is movable for an acceleration measurement, and that is maintained in a rest position by a spring-loaded suspension mount in the absence of inertial forces, and that forms a part of a measurement capacitor. 
     
     
       12. A readout circuit and sensor system for capacitively measuring sensors, comprising: in an input stage of a Σ-Δ modulator, a bridge circuit having four capacitors;   said bridge circuit having two inputs and two outputs;   in each pair of one of said two inputs and one of said two outputs, respectively one of the four capacitors is connected between the input and the output of said each pair;   switches and a clock control with which two different potentials are applied in temporally alternating fashion to the two inputs of the bridge circuit;   two sensor elements of a common type, each forming a measurement capacitor, that form a sensor;   capacitances of said measurement capacitors changing in a common direction, given a change in a quantity to be measured;   said measurement capacitors connected as two of the four capacitors of the bridge circuit; and   the measurement capacitors connected with different inputs and different outputs of the bridge circuit.   
     
     
       13. The readout circuit and sensor system according to claim 12, wherein two reference elements of a common type are present, each of which forms a reference capacitor whose capacitance does not change given a change in a quantity to be measured,   wherein said reference elements are structured similarly to the sensor elements, and are fixed such that capacitances of the reference capacitors are at least approximately equal to capacitances of the measurement capacitors in a predetermined basic state of the sensor elements, and   wherein said reference capacitors are connected as two of the four capacitors of the bridge circuit.   
     
     
       14. The readout circuit and sensor system according to claim 12, wherein the sensor is a pressure sensor, wherein each sensor element has a membrane that is at least region-by-region electrically conductive, which membrane is deformable for a pressure measurement, and which is accessible with one surface to a medium whose pressure is to be measured, and which forms a part of a measurement capacitor. 
     
     
       15. The readout circuit and sensor system according to claim 12, wherein the sensor is an acceleration sensor, wherein each sensor element has an at least region-by-region electrically conductive frame part that is movable for an acceleration measurement, and that is maintained in a rest position by a spring-loaded suspension mount in the absence of inertial forces, and that forms a part of a measurement capacitor. 
     
     
       16. A readout circuit and sensor system for capacitively measuring sensors, comprising: in an input stage of a Σ-Δ modulator, a bridge circuit having four capacitors;   said bridge circuit having two inputs and two outputs;   in each pair of one of said two inputs and one of said two outputs, respectively one of the four capacitors is connected between the input and the output of said each pair; switches and a clock control with which two different potentials are applied in temporally alternating fashion to the two inputs of the bridge circuit;   two sensor elements of a common type, each forming a measurement capacitor, that form a sensor;   capacitances of said measurement capacitors changing in opposite directions, given a change in a quantity to be measured;   said measurement capacitors connected as two of the four capacitors of the bridge circuit; and   the measurement capacitors connected to a common output of the bridge circuit.   
     
     
       17. The readout circuit and sensor system according to claim 16, wherein two reference elements of a common type are present, each of which forms a reference capacitor whose capacitance does not change given a change in a quantity to be measured,   wherein said reference elements are structured similarly to the sensor elements, and are fixed such that capacitances of the reference capacitors are at least approximately equal to capacitances of the measurement capacitors in a predetermined basic state of the sensor elements, and   wherein said reference capacitors are connected as two of the four capacitors of the bridge circuit.   
     
     
       18. The readout circuit and sensor system according to claim 16, wherein the sensor is a pressure sensor, wherein each sensor element has a membrane that is at least region-by-region electrically conductive, which membrane is deformable for a pressure measurement, and which is accessible with one surface to a medium whose pressure is to be measured, and which forms a part of a measurement capacitor. 
     
     
       19. The readout circuit and sensor system according to claim 16, wherein the sensor is an acceleration sensor, wherein each sensor element has an at least region-by-region electrically conductive frame part that is movable for an acceleration measurement, and that is maintained in a rest position by a spring-loaded suspension mount in the absence of inertial forces, and that forms a part of a measurement capacitor.